1. Field of the Invention
The present invention relates to a method for accomplishing audio-video multiplex transmission through a pair of telephone communication cables by multiplexing simultaneous two-way voice signals and a frequency-modulated carrier of monochrome/color signal,
2. Description of the Prior Art
The conventional method of baseband-transmitting simultaneously two-way voice signals through a pair of telephone communication cables and a video signal with power supply through co-axial cable, respectively, is employed in a door-phone camera and so forth. As will be understood from the spectrum distribution of NTSC (National Television System Committee) signal as shown in FIG. 1, many times a video signal will often include a low frequency spectrum having at its lower end a DC component. In this diagram, S1 represents the spectrum of a luminance signal and S2 represents a chrominance subcarrier.
When audio and video signals are directly multiplexed according to the conventional method of transmission, a disadvantage arises. The disadvantage is that the low-frequency component of the video signal overlaps with the audio signals in baseband transmission. For the purpose of preventing such spectra of audio and video signals from overlapping, a carrier is frequency-modulated by the video signal so as to avoid any video component appearing in its low-frequency range and then this modulated video carrier and the voice signals are multiplexed for simultaneous transmission. FIG. 2 graphically shows a spectrum distribution of a carrier that is directly frequency-modulated by the NTSC signal.
At an output of a demodulator, as shown in FIG. 3, the 2nd harmonic component S4 of the FM carrier spectrum is superposed on the demodulated video signal S3. This wide-band unless component S4 can be removed by means of a low-pass filter so that only the required video signal S3 is obtained.
However, if the carrier frequency of the video signal is set too low, the lower side of the 2nd harmonic component S4 of the FM carrier will overlap the upper-side component of the demodulated video signal at the output of the demodulator to consequently bring about beat interference. Since it is necessary to set the carrier frequency as high as 4.5 to 5.5 MHz, according to the conventional method of transmission, it follows that the widening of the bandwidth necessary for this FM carrier transmission will eventually raise a problem in multichannel transmission or the like.
There is also conventional means for baseband transmission of a video signal along a telephone communication cable. Here the telephone communication cable is not only limited to an inhouse cable alone (for example, a communication cable composed of a pair of parallel or twisted wires each having a diameter of 0.65 mm and coated with polyvinyl chloride), but also to an interphone it includes parallel vinyl lines generally used for supplying AC power (composed of a pair of twisted wires (20/0.18 or 30/0.18)). These various communication cables will be referred to simply as cable. A problem with using such cable is that the cable is harmfully affected by external radio waves, and particularly, medium waves (550-1350 kHz) which have been induced along the cable to cause beat interference and so forth on a reproduced picture at the receiving end. That is, on a television picture, there will appear beat pattern if any interference wave superposed on the video signal is more than -50 dB, whereby the picture quality will deteriorated. In order to eliminate "normal mode" interference, a push-pull transmission with balancers may be preferable, but it is still difficult to completely prevent "common mode" disturbance caused by medium waves having high field-intensity. Furthermore, due to the necessity of fixing the cable with staples or threading the cable through an iron pipe or so forth, it is unavoidable that some effects are actuated with respect to the environmental condition for the cable, hence causing multipath reflection of the carrier being transmitted through the cable. Particularly, transmission characteristics of a span of cable over 100 meters indicate a uniform descending curve at high frequencies occasionally with some dips of -6 dB or so at medium frequencies (0.5 to 2 MHz) caused by impedance mis-matching the cable's terminators. It may be contrived to transmit the video signal by the use of a co-axial cable for the purpose of obtaining satisfactory reproduced picture. However, for example, when replacing existing equipment for interphone with new equipment including a door phone camera to pick up visitors, there still remains a disadvantage of requiring additional work for installing another separate co-axial cable besides the existing one.